note the offset of the carboxyl ¹³C nucleus signal of acetic acid. Finally load a pulse program for c.w. decoupling. You have to set:

decoupler attenuation, to be varied
td	2 k
sw	100 Hz
o1	on resonance for carboxyl ¹³C nucleus of acetic acid
o2	25 Hz offset from the ¹H resonance of the CH₃ protons of acetic acid
p1	45° ¹³C transmitter pulse
d1	2 s
ns	1

Record the ¹³C quartet of the carboxyl signal of acetic acid using 100 Hz spectral width and the transmitter offset directly on resonance; the decoupler should be off for the first experiment. Measure the spin coupling constant from the separation of the two inner lines of the quartet. Adjust the decoupler offset to 25 Hz from the ¹H resonance of the methyl group of acetic acid and record the ¹³C spectra with different decoupler attenuations. Measure the frequency separation of the two inner lines of the quartet and from this calculate the decoupler field strength γB₂ [Hz] using Equation (1)

γB₂ = [(J Δ / J_R)² + 0.25 (J_R² − J²) − Δ²]^1/2(1)

where J is the unperturbed spin coupling constant (6.63 Hz), J_R the reduced splitting of the inner lines of the quartet and Δ the offset of the decoupler frequency from the ¹H resonance.

5. Processing

Use standard 1D processing as described in Experiment 3.2, with lb = 0.3 Hz, apply zero-filling to ensure enough data points for the reduced splittings.

6. Result

A typical set of values for Δ = 25 Hz obtained on an ARX-200 spectrometer is given below, from which the graph was calculated:

Dec. att. [dB]	80	77	74	71	68	65	62	59
J_R [Hz]	6.6	6.54	6.4	6.2	5.65	5.05	4.45	3.48